This invention relates to an information encoding method and apparatus and an information decoding method and apparatus for encoding and decoding signals of plural channels employed in a multi-channel sound acoustic system, such as a motion picture film projection system, a video tape recorder or a video disc player. The invention also relates to a recording medium on are recorded information signals formed by the information encoding method and apparatus.
In a motion picture film on which are recorded audio or speech signals of plural channels, there are occasions wherein digital audio signals of five channels, namely a left channel (L), a center channel (C), a right channel (R), a surround left channel (SL) and a surround right channel (SR), are handled.
Since a recording medium or a transmission route of a large capacity is required if the signals of the respective channels are directly recorded on a recording medium or transmitted on a communication route, these signals are customarily encoded by high-efficiency encoding for recording or transmission.
There exist a variety of systems for high efficiency encoding of audio or speech signals. Among these systems, there is known a so-called transform coding in which time-domain audio signals are divided into unit time blocks, the block-based time-domain signals are converted by orthogonal transform into frequency domain signals, which are then divided into plural frequency bands for encoding from one band to another.
There is also known a sub-band coding or a non-blocking frequency band splitting system in which time-domain audio signals are divided into plural frequency bands for encoding without dividing the time-domain audio signals into unit time blocks.
In another high-efficiency encoding system consisting in combination of the sub-band coding and transform coding, the time-domain signals are divided into plural frequency bands and the signals of the respective bands are converted by orthogonal transform into frequency-domain signals which then are encoded from one band to another.
The apparatus for encoding the plural signals with the aid of the above-described encoding systems for recording or transmission or the apparatus for decoding these encoded signals for reproduction or reception is usually in need of a number of encoding and decoding circuits equal to the number of the channels. In the case of the above-mentioned motion picture film, encoding and decoding circuits for five channels are required.
FIG. 1 shows a construction of an encoding apparatus (encoder) for independently encoding digital audio signals of the above-mentioned five channels L, SL, C, SR and R from channel to channel.
Referring to FIG. 1, digital audio signals of the left channel (L) and digital audio signals of the surround left channel (SL) enter an input terminal 51.sub.1 and an input terminal 51.sub.2, respectively. In a similar manner, signals of the center channel (C), surround right channel (SR) and the right channel (R) enter input terminals 51.sub.3, 51.sub.4 and 51.sub.5, respectively.
The digital audio signals of these plural channels are sent via the input terminals 51.sub.1 to 51.sub.5, associated with the respective channels, to encoding circuits 52.sub.1 to 52.sub.5, similarly associated with the respective channels, respectively. The signals encoded by these encoding circuits 52.sub.1 to 52.sub.5 are sent to a codestring generating circuit 53.
The codestring generating circuit 83 assembles the encoded signals of the plural channels into a codestring, in accordance with a pre-set format, fop transmission or recording on the recording medium.
FIG. 2 shows the construction of a decoding apparatus (decoder) for decoding the recorded or transmitted encoded signals of the plural channels.
Referring to FIG. 2, the encoded signals of the codestring, reproduced from a recording medium or transmitted over a transmission path, are supplied via an input terminal 61 to a codestring separating circuit 82. The codestring separating circuit 62 separates the supplied codestring signals into encoded signals of the respective channels in accordance with a pre-set format. The encoded signals of the respective channel are sent to decoding circuits 63.sub.1 to 63.sub.5 provided for the respective channels. The encoded left-channel signals are decoded by a decoding circuit 63.sub.1, while the encoded surround-left channel signals are decoded by a decoding circuit 63.sub.2. In a similar manner, the encoded center channel, surround right channel and the right channel signals are decoded by the decoding circuits 63.sub.3, 63.sub.4 and 63.sub.5, respectively. The digital audio signals of the respective channels L, SL, C, SR and R, decoded by these decoding circuits 63.sub.1 to 63.sub.5, are outputted via associated output terminals 64.sub.1 to 64.sub.5. For decoding and reproducing codestring signals, obtained on encoding signals of respective channels, the number of the reproducing apparatus, such as speakers and amplifiers, equal to the number of the channels, are required. In the above example, the reproducing components of five channels are required.
If a reproducing apparatus of a simplified structure is employed, there ape occasions wherein it is required to reproduce the signals over the number of channels smaller than the number of the original channels, for example, over two channels. In such case, it is necessary to mix the playback signal of the respective channels, decoded by the 5-channel decoder described above, into two channels. This may be realized by addition as shown by the equations (1) and (2): EQU Lm=L+a*C+b,SL (1) EQU Rm=R+a*C+b*SR (2)
where Lm, Rm, L, R, C, SL and SR denote a mixed left channel, a mixed right channel, a left channel, a right channel, a center channel, a surround left channel and a surround right channel, respectively. Also, a and b denote mixing coefficients.
In such case, while it is sufficient to reproduce only two channels, decoding circuits for five channels are required as a decoder. Since the scale of the decoder is not changed from the above-mentioned decoder for the five channels, such decoder cannot be realized easily.
For easily reproducing two-channel signals, it may be contemplated to transmit signals obtained on pre-mixing signals of channels other than the inherent channels on the two channels for transmitted signals and to decode only the signals of these two channels by the decoder. The method of pre-mixing signals of pre-set channels is termed a pre-mix method.
For example, the encoder forms signals of respective channels by the equations (3) to (7): EQU Lm=L+a*C+b*SL (3) EQU Rm=R+a*C+b*SR (4) EQU C (5) EQU SL (6) EQU SR (7)
Thus it is sufficient for the decoder to have a decoding circuit for two channels, that is a mixed left channel (Lm) and a mixed right channel (Rm), so that the decoder can be realized easily.
On the other hand, for reproducing signals of e.g., the left channel (L) and the right channel (R), signals of the channels not transmitted independently need to be produced by subtracting these signals from signals of the mixed channels, that is the mixed left channel and the mixed right channels. That is, signals transmitted independently are subtracted from the signals of the mixed channels. This operation is termed a dematrixing operation.
For example, for producing signals of the left channel (L) and the right channels (R) from the signals of the equations (3) to (7), it is necessary to perform the operations of the equations (8) and (9): EQU L=Lm-a*C-b*SL (8) EQU R=Rm-a*C-b*SR (9)
FIG. 3 shows a construction of an encoder for encoding the pre-mixed channel signals. In FIG. 3, the components having the similar function to that of FIG. 1 are depicted by the same reference numerals.
Referring to FIG. 3, an addition circuit 72.sub.1 is arranged upstream of the encoding circuit 52.sub.1 provided in FIG. 1 for a left channel. The addition circuit 72.sub.1 is fed with left-channel signals entering the input terminal 51.sub.1, a signal obtained by multiplying the surround left channel signal entering the input terminal 51.sub.2 by the coefficient b by a multiplication circuit 71.sub.2 and a signal obtained by multiplying the center channel signal entering the input terminal 51.sub.3 by the coefficient a. These signals are mixed by the addition circuit 72.sub.1 to form signals of the mixed left channel Lm. These mixed left channel Lm are fed to the encoding circuit 52.sub.1 to form encoded signals which are sent to a codestring generating circuit
On the other hand, an addition circuit 72.sub.2 is arranged upstream of an encoding circuit 52.sub.5 provided in FIG. 1 for a right channel. The addition circuit 72.sub.1 is fed with right-channel signals entering the input terminal 51.sub.5, a signal obtained by multiplying the surround right channel signal entering the input terminal 51.sub.3 by the coefficient a by a multiplication circuit 71.sub.1 and a signal obtained by multiplying the center channel signal entering the input terminal 51.sub.3 by the coefficient a by a multiplication circuit 71.sub.1. These signals are mixed by the addition circuit 72.sub.2 to form signals of the mixed right channel Rm. These mixed right channel Rm ape fed to the encoding circuit 52.sub.5 to form encoded signals which are sent to the codestring generating circuit 53.
The signals of the surround left channel, center channel and the surround right channel, entering the input terminals 51.sub.2 to 51.sub.4, respectively, are encoded by associated encoding circuits 52.sub.2 to 52.sub.4, and thence supplied to the codestring generating circuit 53.
The codestring generating circuit 53 generates the codestring of the pro-set format, from the signals encoded by the encoding circuits 52.sub.1 to 52.sub.5, and outputs the signals of the codestring at the terminal 54.
FIG. 4 shows an arrangement of a decoder for decoding and reproducing only signals of two mixed channels, that is the mixed left channel and the mixed right channel, from the codestring containing signals of the channel obtained by pre-mixing by the arrangement of FIG. 3. In FIG. 4, the components having the same function as that of similar components shown in FIG. 2 are depicted by the same reference numerals.
In FIG. 4, only signals of the mixed left channel and the mixed right channel are taken out from the codestring signals entering the input terminal 61 so as to be sent to the associated decoding circuits 63.sub.1 and 63.sub.2. The signals decoded by the decoding circuit 63.sub.1 are outputted at the output terminal 64.sub.1 as signals of the mixed left channel Lm, while the signals decoded by the decoding circuit 63.sub.2 are outputted at the output terminal 64.sub.2 as signals of the mixed right channel Rm.
FIG. 5 shows an arrangement of a decoder for decoding the codestring signals comprised of the pre-mixed channels and dematrixing the codestring signals to the non-pre-mixed state. In FIG. 5, the components having the same function as that of similar components shown in FIG. 2 are depicted by the same reference numerals.
Referring to FIG. 5, the codestring signals entering the input terminal 61 are separated by the codestring separating circuit 62 into encoded signals of the mixed left channel, surround left channel, center channel, surround right channel and the mixed right channel. The encoded signals of the mixed left channel are decoded by the decoding circuit 63.sub.1, while the encoded signals of the surround left channel are decoded by the decoding circuit 63.sub.2. In a similar manner, the encoded signals of the center channel, right surround channel and the mixed right channel are decoded are decoded by the decoding circuits 63.sub.3, 63.sub.4 and 63.sub.5, respectively. The decoded signals of the surround left, center and right surround channels are outputted at associated output terminals 64.sub.2, 64.sub.3 and 64.sub.4, respectively. An addition circuit 82.sub.1 is arranged downstream of the decoding circuit 63.sub.1 configured for decoding the mixed left channel. This addition circuit 82.sub.1 is supplied with the signals of the mixed left channel, decoded by the decoding circuit 63.sub.1, as an addition signal. The addition circuit is also supplied with a signal obtained on multiplying the surround left channel signal decoded by the decoding circuit 63.sub.2 with the coefficient b, by the multiplication circuit 81.sub.2, as a subtraction signal, and with a signal obtained on multiplying the center channel signal decoded by the decoding circuit 63.sub.3 with the coefficient a by a multiplication circuit 81.sub.1, as a subtraction signal. Thus the addition circuit 82.sub.1 generates a signal obtained on subtracting the surround left channel signal and the center channel signal from the signal of the mixed left channel, that is the left channel signal. This left channel signal is outputted at the output terminal 64.sub.1.
On the other hand, an addition circuit 82.sub.2 is arranged downstream of the decoding circuit 63.sub.5 configured for decoding the mixed right channel. This addition circuit 82.sub.2 is supplied with the signals of the mixed right channel, decoded by the decoding circuit 63.sub.5, as an addition signal. In addition, the addition circuit is supplied with a signal obtained on multiplying the surround right channel signal decoded by the decoding circuit 63.sub.4 with the coefficient b by a multiplication circuit 81.sub.3, as a subtraction signal, and with a signal obtained on multiplying the center channel signal decoded by the decoding circuit 63.sub.3 by the coefficient a by the multiplication circuit 81.sub.1, as a subtraction signal. Thus the addition circuit 82.sub.2 generates a signal obtained on subtracting the surround right channel signal and the center channel signal from the signal from the mixed right channel, that is the right channel signal. This right channel signal is outputted at the output terminal 64.sub.5.
The above-described system represents a technique employed in the MPEG2 audio standard (ISO/IEC 13813-3) by the Moving Picture Image Coding Experts Group (MPEG) for encoding moving pictures for storage installed under the auspices of the International Organization for Standardization (ISO).
Meanwhile, signals reproduced on dematrixing tends to be increased in noise as compared to the original signals. This noise is provisionally termed the mixing noise.
It is assumed that the signal level of the left channel L in the equation (3) is significantly smaller than the signal level of other channels (a*C) or (b*SL). If the signal mixed with the other channels is encoded in accordance with the high efficiency encoding system and subsequently decoded and dematrixed, the quantization noise of the other channels becomes predominant such that the left channel signal obtained by the calculation of the equation (8) superimposed by the quantization noise of the other channels is issued as an output signal.
Examples of the increased noise are explained with reference to waveform signals of the respective channels shown in FIG. 6.
FIG. 6A shows examples of the signal waveforms of the channels L, R, C, SL and SR entering the input terminals 51.sub.1 to 51.sub.5 of FIG. 3. FIG. 6B shows signal waveforms of the respective channels shown in FIG. 6A which are pre-mixed by the configuration of FIG. 3 and are subsequently supplied to the encoding circuits 52.sub.1 to 52.sub.5. That is, Lm and Rm in FIG. 6B stand for the signal waveforms of the mixed left channel and the mixed right channel, respectively. The signal waveforms of the remaining three channels are the same as those shown in FIG. 6A. FIG. 6C shows examples of signal waveforms of the respective channels L, R, C, SL and SR obtained by sending the signals of the respective channels shown in FIG. 6B to the configuration of FIG. 5 for decoding by the associated decoding circuits 63.sub.1 to 63.sub.5 and for dematrixing in the manner as shown in FIG. 5, that is examples of the signal waveforms outputted by the output terminals 64.sub.1 to 64.sub.5.
Comparison of the signal waveforms of the respective channels of FIG. 6A to those of FIG. 6C reveals that the decoded and dematrixed signals of the left channel L and the right channel R of FIG. 6C contain signal components not present in the encoded signal of the respective channels shown in FIG. 6A. This noise is the mixed noise generated by dematrixing as a result of superimposition of the quantization noises of the other channels.
Such phenomenon of noise generation is known to occur most significantly with signals of the channels having a small signal level.
For overcoming such inconvenience, it has been contemplated to selectively switch the respective channels of small signal levels for independent coding.
There is also known a system which is known as a simulcasting system in which the number of channels is increased and which consists in encoding signals of all the independent channels and signals of mixed channels.
FIG. 7 shows an encoder consisting in encoding signals in accordance with the simulcast system. In FIG. 7, the components having the function similar to that shown in FIGS. 1 or 3 are indicated by the same reference numerals.
Referring to FIG. 7, the left channel signals supplied to the input terminal 51.sub.1 are sent to the encoding circuit 52.sub.2 and to the addition circuit 72.sub.1. The signals encoded by the encoding circuit 52.sub.2 are sent to the codestring generating circuit 53.
On the other hand, the addition circuit 72.sub.1 is fed with the left channel signal from the input terminal 51.sub.1. The addition circuit 72.sub.1 is also fed with a signal obtained by multiplying the surround left channel entering the input terminal 51.sub.2 with the coefficient b by the multiplication circuit 71.sub.2 and with a signal obtained by multiplying the center channel entering the input terminal 51.sub.3 with the coefficient a by the multiplication circuit 71.sub.1. These signals are mixed together by the addition circuit 72.sub.1 so as to form signals of the mixed left channel Lm. These mixed left channel signals are sent to and encoded by the encoding circuit 52.sub.1 so as to be sent to the codestring generating circuit.
The right channel signals supplied to the input terminal are sent to an encoding circuit 52.sub.6 and to the addition circuit 72.sub.2. The signals encoded by the encoding circuit 52.sub.2 are sent to the codestring generating circuit 53.
On the other hand, the addition circuit 72.sub.2 is fed with the right channel signals from the input terminal 51.sub.5. The addition circuit 72.sub.2 is also fed with a signal obtained by multiplying the surround right channel signal entering the input terminal 51.sub.4 with the coefficient b by the multiplication circuit 71.sub.3 and with a signal obtained by multiplying the center channel entering the input terminal 51.sub.3 with the coefficient a by the multiplication circuit 71.sub.1. These signals are mixed together by the addition circuit 72.sub.2 so as to form signals of the mixed right channel Rm. These mixed right channel signals are sent to and encoded by a encoding circuit 52.sub.7 so as to be sent to the codestring generating circuit 53.
The surround left channel signals and the surround right channels, entering the input terminals 51.sub.2 to 51.sub.4, are encoded by associated encoding circuits 52.sub.3 to 52.sub.5 before being sent to the codestring generating circuit 53.
The codestring generating circuit 53 generates the above-mentioned codestring of the pre-set format from the signals encoded by the above-mentioned encoding circuits 52.sub.1 to 52.sub.7. This codestring is outputted at the terminal 54.
Thus, with the encoder shown in FIG. 7, the mixed left and mixed right channel signals are encoded along with signals of the other independent channels.
The simplified decoder for decoding only signals of the two mixed channels, associated with the encoder shown in FIG. 7, is of the same configuration as that shown in FIG. 4 and hence is not explained for simplicity.
The decoder for decoding the encoded signals of the simulcast system is the same as that shown in FIG. 3 and hence is not explained specifically for simplicity. That is, with the configuration shown in FIG. 3, even if the codestring signal obtained by the configuration of FIG. 7 is supplied at the input terminal 61, the mixed left and mixed right signals are not decoded.
The above-described technique of switching the contents of the independent encoding channels as means for reducing the mixing noise produced on dematrixing the signal of the mixed channels is effective if the signals to be mixed together are of a low signal level. However, if the signal is higher in level and has no correlation with other channels, the mixed noise tends to be heard. On the other hand, for implementing this technique, the operation of detecting the signal level or of switching the channels becomes complex and the circuit scale is increased, while a switching control signal specifying which channel has been selected needs to be transmitted to the decoder.
If, as means for decreasing the mixed noise generated on dematrixing the mixed channel signals, the technique of increasing the number of channels and encoding the independent channels and the mixed channels, the problem of deteriorated sound quality is raised due to shortage of the number of bits caused by increase in the number of channels. Since the number of bits needs to be increased for prohibiting the sound quality from being lowered, the above-mentioned technique is inappropriate for a recording medium having a limited volume of the recordable information.